The instant invention relates to a laser for generating narrow-band radiation, comprising
a laser resonator which includes two reflecting elements and a laser active medium disposed between them,
a group of several refractive, wavelength selective elements each deflecting incident light at an angle which is specific of the wavelength of the incident light.
A laser of this kind is known from U.S. Pat. No. 5,150,370.
Lasers of the kind mentioned are applied particularly in the art of photographic lithography for the production of integrated circuits. Wavelengths below 200 nm are required for making structures in the dimension range of 0.18 xcexcm by photographic lithography. Achromatic imaging optics for this wavelength range are difficult to produce. For this reason radiation of very narrow bandwidths is needed for the photolithographic production of structures of the above mentioned order of magnitude in order to keep errors caused by chromatic aberration in the structures obtained through photographic lithography within admissible tolerance limits. Acceptable bandwidths for refractive imaging optics lie in the range of 1 pm (refractive optics made of quartz alone: 0.3 pm, refractive optics of different materials: a few pm), the range extending from 10 pm to 100 pm for the catadioptrics which operate with a beam splitter and mirrors. In contrast thereto, an ArF excimer laser (xcex=193 nm) has a bandwidth of 430 pm in the so-called free running operation so that optical elements must be provided inside the laser to limit the bandwidth if the above requirements are to be met.
It is known in the art to locate gratings providing beam expansion, prism assemblies, and/or Fabry-Perot etalons in the beam path of the resonator for narrowing the bandwidth.
As gratings and Fabry-Perot etalons are characterized by a high loss factor per round trip (transmission  less than 70%) they are suited only conditionally for use in an ArF laser of which the resonator loss is much greater than in an XeCl or KrF laser, for instance. With Brewster prisms, on the other hand, and especially prisms with an antireflective coating the transmission is more than 95%.
Arrangements thus have been suggested with one or two dispersion prisms disposed in the resonator of the laser (1st International Symposium on 193 nm Lithography, Colorado Springs, Aug. 15-18, 1995). In this manner, bandwidths of approximately 10 pm can be achieved. The principle of such an arrangement is illustrated diagrammatically in FIG. 1 which depicts part of the beam path of the resonator. The incoming beam passes through an (optional) achromatic beam expander 1 and then is directed through a dispersion prism 2 to the highly reflective mirror 3 of the resonator which in turn reflects the incident light through the prism 2 and the beam expander 1 back to the second mirror of the resonator. The resonator mirror 3 is disposed at an angle with respect to the optical axis A of the laser, this angle corresponding to the deflection angle xcex3 of the prism for the desired wavelength, e.g. xcex=193 nm so that essentially only light of this wavelength is reflected and circulated in the resonator.
However, in excimer lasers including such prism assemblies time variations of the emission wavelength of the laser occur during burst operation, whereby the usefulness of the laser is limited as far as photographic lithography is concerned.
It is the object of the invention to improve a laser of the kind specified initially such that variations in radiation in response to temperature and, therefore, dependent on time during burst operation, are reduced as best as possible.
According to the solution proposed by the invention, in a laser for generating narrow-band radiation, comprising a laser resonator which includes two reflecting elements and a laser active medium disposed between them, as well as a group of several refractive, wavelength selective elements each deflecting incident light at an angle (xcex3a, xcex3b) which is specific of the wavelength of the incident light, at least one of the refractive, wavelength selective elements has a refractive index which rises as the temperature rises (dn/dT greater than 0) and at least one of these elements has a refractive index which drops as the temperature rises (dn/dT less than 0). The laser may be an excimer laser, especially an ArF excimer laser. The group or refractive, wavelength selective elements preferably are arranged in the beam path of the resonator.
It was found that the variations in time of the emission wavelength, as observed with arrangements according to the state of the art, are due to the fact that the refractive index of the prisms used is dependent upon temperature and, therefore, the deflection angle for the desired radiation, e.g. light of 193 nm wavelength, varies during burst operation because of the radiation absorption in correspondence with the heating and cooling of the prisms. In accordance with the invention, this variation of the deflection angle at least is reduced by use of a combination of no less than two refractive elements which differ in sign of dn/dT.
If the elements are designed and arranged appropriately the temperature responsive changing of the deflection angle can be eliminated altogether. In particular, the angles of incidence at the refractive dispersive elements can be selected such that the sum of the individual deflection angles of the respective elements at a predetermined wavelength of the incident light is independent of temperature variations during operation of the laser.
For the ultraviolet range of the spectrum, quartz glass is the preferred material for at least one refractive dispersive element, especially quartz glass known under the tradename of Suprasil (dn/dT greater than 0), while CaF2 (dn/dT less than 0) is the material for at least one refractive dispersive element.
A special embodiment of the invention comprises a beam splitter which is disposed between the laser active medium and one of the reflecting elements, the group of refractive, wavelength selective elements being arranged between the beam splitter and the reflecting element. In this way the assembly which confines the bandwidth is positioned in a part of the resonator which, in operation, has a relatively low light intensity as a consequence of which the service life of the assembly is prolonged.
The refractive, dispersive elements particularly may be dispersion prisms. At the same time, preferably at least one of these prisms is a Brewster prism. The surface of incidence of the other prisms may be given an antireflective coating.
The laser according to the invention especially may comprise a first dispersion prism and a second dispersion prism, the change in response to temperature of the refractive index of the first prism having a different sign than the change of the refractive index of the second prism, and the apex angle of the second prism being selected such that upon incidence of light radiating through the first prism on the second prism at the Brewster angle, the overall deflection angle as determined by the first and second prisms is independent of temperature variations during operation of the laser. This eliminates the need for an antireflective coating on the surface of incidence of the second prism.
Advantageously, the surfaces of incidence of the prisms are illuminated as completely as possible. To accomplish that, a beam expander may be arranged ahead of the first prism.
In another embodiment of the invention a Fabry-Perot etalon may be included in the beam path of the laser in front of or behind the group of refractive dispersive elements. Bandwidths of less than 1 pm are obtainable with this embodiment.